1. Field of the Invention
The present invention relates to a device for detecting malfunction in a system for purging gas evaporated from a fuel tank of an automotive vehicle.
2. Description of Related Art
In a usual evaporated gas control system, fuel gas evaporated from a fuel tank is absorbed to a canister, and the absorbed gas is sucked into a downstream portion of a throttle valve by negative pressure generated in an intake air passage. In this manner, the evaporated gas absorbed to the canister is purged. JP-A-5-187332 discloses a device for detecting malfunction in the evaporated gas purging system. In this detecting device, the malfunction in the purging system is detected based on changes in an opening degree of an idle speed control valve when an opening degree of a purge control valve is compulsory changed while an engine is idling.
On the other hand, there is another type of evaporated gas control system which is used in an engine having a supercharger. In this type of engine, pressure in an air intake passage at a downstream portion of a throttle valve becomes positive when intake air is compulsorily supplied by a compressor of a supercharger. In particular, pressure at the down stream portion of the throttle valve becomes positive when the engine is operated at a high speed and under a high load. In this case, it becomes impossible to purge the evaporated gas by means of the negative pressure at the downstream portion of the throttle valve.
In order to make it always possible to purge the evaporated gas, an engine having a purging passage that is branched out to an upstream purging passage and a downstream purging passage is proposed. The upstream purging passage is connected to the upstream portion of the throttle valve, while the downstream purging passage is connected to the downstream portion of the throttle valve. When the pressure at the downstream portion of the throttle valve is positive, the evaporated gas is purged through the upstream purging passage by utilizing a small amount of negative pressure in the upstream purging passage generated by a pressure loss in an air cleaner positioned at an upstream end of the intake air passage.
Though JP-A-5-187332 discloses a device for detecting malfunction in the evaporated gas purging system having one purging passage, i.e., the downstream purging passage, it does not offer or suggest how to detect malfunction in the upstream purging passage.